1. Field of the Invention
This invention relates to an ink composition for a printer, and more particularly to a non-aqueous ink composition containing therein non-volatile solvent as a main material.
2. Description of the Related Art
Ink jet recording has been rapidly put into wide use since it realizes high-speed printing with relatively less noise. In an ink jet printer, highly flowable liquid ink is injected onto a recording medium through a fine nozzle, and the ink jet printers are broadly divided into an on demand type and a continuous type.
Inks for ink jet printers are broadly divided into an aqueous type and a non-aqueous type. The aqueous type inks generally comprise water-soluble dye such as acid dye, direct dye and basic dye dissolved in glycol solvent and water, and there has been a problem that ink jet printings by an aqueous type ink are generally poor in resistance to water.
On the other hand, non-aqueous type inks are excellent in resistance to water. The non-aqueous type inks are broadly divided into solvent type inks and oil type inks. Especially, various kinds of oil type inks have been proposed since the oil type ink is advantageous as ink for high-speed ink jet printer in that it is less apt to cause clogging of the nozzle and requires less frequent cleaning during printing.
For example, in Japanese Unexamined Patent Publication No. 57(1982)-10661, there is disclosed on oil type ink in which specific dispersant and fine pigment particles are dispersed in non-aqueous solvent such as aliphatic or aromatic hydrocarbon series solvent, halogenated hydrocarbon series solvent, ether series solvent, ketone series solvent, ester series solvent, polyhydric alcohol or the like. However this ink is disadvantageous in that it is not sufficiently satisfactory in shelf stability and it is difficult to keep sufficient discharge stability unless the pigment particles are further atomized since the inner diameter of the nozzle becomes smaller in order to meet demand for higher resolution.
As an ink jet ink which is excellent in discharge stability from a nozzle, there has been disclosed, in Japanese Unexamined Patent Publication No. 11(1999)-279467, an ink which is 5 to 15 pcs in viscosity at 250° C. and in which organic pigment atomized by kneading a mixture of at least three components including organic pigment, water-soluble inorganic salt in an weight not lighter than three times the weight of the organic pigment and water-soluble solvent is dispersed in high-boiling solvent together with suitable dispersant.
However, though the ink jet ink disclosed in Japanese Unexamined Patent Publication No. 11(1999)-279467 is improved in some extent in resistance to clogging and/or ink discharge stability in the nozzle or in the ink supply path, it is not sufficiently satisfactory in shelf stability and agglomeration of pigment particles occurs with time, which result in deterioration in dispersion of pigment particles.
When shelf stability of ink is unsatisfactory, discharge of ink becomes instable and condensation of pigment particles in the ink becomes insufficient, which results in a drop out of a part of the image, blur of the image and/or an insufficient image density (especially a solid part of the image). Further, when ink stored for a long time is used, discharging conditions (e.g., the voltage to be applied to the ink jet), the rate at which the ink is condensed and the like can be different from when fresh ink is used, which means that the recorded images change with the state of storing the ink.